This invention is generally related to parenteral fluid warming systems and, more particularly, to the structure of a fluid warming cassette used with a warming apparatus, in which the structure of the cassette includes a fluid container with rails and a stiffening member near the proximal end of the cassette.
Fluid warming apparatuses, designed to warm and administer parenteral fluids and blood products (hereinafter “fluids”), are in common use. Generally, such fluids are heated and administered using fluid warming systems. A parenteral fluid warming system usually includes a warming unit into which a fluid warming cassette is placed. The fluid warming cassette includes a fluid container with a structure designed for handling and for being received and supported in the warming unit. The fluid container includes a fluid channel and is typically made of plastic film material and/or thin metal.
In use, the cassette is placed into the warming unit to heat the fluids as they flow through the fluid channel. Heat is transferred to the fluid through the fluid container from a heat source such as heated metal plates, heated liquid, or heated gas. Metal plate, “dry heat” exchanger warming units are widely known.
In such systems, heat transfer from the warming unit to the fluid warming cassette is typically by conduction, with the heat source including, for example, one or more metal plates brought into contact with the warming cassette. There are systems that operate by conduction between a bath of heated water and a fluid warming cassette; some systems use induction for heating; others use micro-waving means. Still other systems operate by convection, disposing a fluid warming cassette in a flow of heated air. Those skilled in the art will appreciate that heat transfer in such systems is a complex process that actually compounds conduction, radiation, and convection. Therefore, characterization of a mode of heat transfer to a fluid warming cassette actually denotes the principal mode of transfer and does not necessarily exclude contribution by one or more additional modes.
To increase the thermal efficiency and temperature responsiveness of a fluid warming system in which a fluid warming cassette is disposed for conductive heat transfer from warming plates, the distance between the heater plates is usually very small. This implies that a fluid warming cassette should be a thin, flat container, constructed from selected materials. Plastic film materials are commonly used in the manufacture of disposable fluid warming containers. Such designs are disclosed in the four above-referenced patent applications.
Since these fluid containers are thin, it would be difficult to insert one into a conductive warming unit simply by sliding it between the warming plates. The container may kink or tear when being slid into or out of such a narrow space. As a result, the fluid container needs some type of structural support, usually a frame, although a tensioning rod may also be used. A frame stiffens the fluid container so that the fluid warming cassette can be handled, and also inserted into and removed from the warming unit. Together, the fluid container and frame constitute a fluid warming cassette, a modular unit of equipment designed to be received in, or inserted into, a larger warming unit. A flexed tensioning rod disposed in a fluid container exerts a tension between the sides of the container thereby imparting the desired stability and handleability to a fluid warming cassette.
A frame for a fluid warming cassette comprehends numerous elements, including a quadrilateral frame structure with a handle portion that extends outside of the warming unit when the cassette is seated in the unit in order to provide something that can be grasped to manipulate the cassette for insertion and extraction. The frame type of construction adds steps to the process for manufacturing fluid warming cassettes, leading to lower yields and higher manufacturing costs. In this regard, the frame is preferably made by a vacu-form process which can yield frames with dimensional variations that are not accommodated by the narrow space between warming plates. Relatively expensive materials must be used for tensioned rods in order to achieve acceptable shelf lifetimes for fluid warming cassettes which incorporate them.
It would be advantageous if a simplified, effective, low cost fluid warming cassette were made available for a parenteral fluid warming system in which the cassette is received between heating plates of a warming unit for conductive heat transfer therefrom.
It would be advantageous if a fluid container could be invested with enough stiffness for insertion between close-set warming plates of a warming unit, yet be thin enough to efficiently transfer heat by conduction from the plates to the fluid, without the requirement of an expensive frame.
It would be advantageous if the fluid warming cassette were provided with a keying mechanism that prevents it from being inserted either upside down, or backwards in a warming unit.